1. Field of the Invention
This invention relates to an adhesion promoter and primer compositions prepared therefrom.
2. Description of Prior Art
Adhering silicone elastomers to various substrates is a continuing problem. There is an ever present need for better adhesion. With the changing substrates and silicone elastomers, the search for new methods of adhering the silicone rubber to substrates and for new materials to provide adhesion continues. One particular area wherein adhesion is required, is the application where a room temperature vulcanizable silicone elastomer is to be adhered to substrates, such as metals. Many of the prior art methods of adhering silicone rubber to substrates either are not suitable for today's materials, provide an insufficient bond strength or are too expensive because of the complex method or materials.
Keil in U.S. Pat. No. 2,751,314 teaches that silicone rubber can be adhered to the surface of a solid by depositing a continuous coating having 50 to 100 percent by weight of an organotitanate and 0 to 50 weight percent alkylpolysilicate, depositing over this coating another coating having 1 to 10 percent by weight alkylpolysilicate and 90 to 99.9 percent by weight of a toluene soluble organopolysiloxane, thereafter applying a silicone rubber such as a room temperature vulcanizable silicone rubber. While this method described by Keil does bond silicone rubber to a substrate, it requires two coatings to provide an adequate bond and it also shows that a silicone rubber layer containing alkylpolysilicate must be deposited over the titanate primer layer to achieve adequate bonding.
Harper in U.S. Pat. No. 2,979,420 teaches that a room temperature vulcanizing silicone rubber can be bonded to a surface by applying certain monoorganotriacyloxysilanes to the surface as a primer. Although this method is simple, the bonding achieved is insufficient in strength for many applications.
Chadha in U.S. Pat. No. 3,498,824 teaches that an alkoxyacyloxysilane can be used as a primer to adhere room temperature vulcanizing silicone rubber to metal substrates. Although the bond strength is improved with this primer compared to no primer, the bond strength is insufficient under many circumstances.
Young in U.S. Pat. No. 3,671,483 teaches that certain compositions are useful as primers for adhering silicone rubber to epoxy resin and polyurethane substrates. The compositions described contain alkylsilicates, organotitanates, solvent and a compound of the formula EQU CF.sub.3 CH.sub.2 CH.sub.2 (CH.sub.3).sub.2 SiOSiX.sub.3
where X is EQU --CH.sub.2 CH.sub.2 Si(OOCCH.sub.3).sub.3
or a hydrogen atom and at least one X is EQU --CH.sub.2 CH.sub.2 Si(OOCCH.sub.3).sub.3.
these primers provide useful adhesion, but have insufficient bond strength and those which contain silicon bonded hydrogen atoms form gas on storage.
Matherly in U.S. Pat. No. 3,714,109 teaches a mixture of an organic solvent, bis(acetylacetonyl)-diisopropyltitanate, an alkylpolysilicate and a mixture containing an organosiloxane having dimethylsiloxane units and methylhydrogensiloxane units and trimethylsiloxane units or dimethylhydrogensiloxane units and a modified organosiloxane having in addition to the above siloxane units, EQU (CH.sub.3 COO).sub.3 SiCH.sub.2 CH.sub.2 (CH.sub.3)SiO
units and EQU (CH.sub.3 COO).sub.3 SiCH.sub.2 (CH.sub.3).sub.2 SiO.sub.0.5
units. These primer compositions are suitable for adhering silicone rubber to substrates in certain applications but have insufficient bond strength for many applications and additionally have a potential to form hydrogen gas because of the presence of silicone-bonded hydrogen atoms.
Lee and Schulz in U.S. application Ser. No. 406,065 filed Oct. 12, 1973, now U.S. Pat. No. 3,914,199, and assigned to the assignee of this application, teach an organic solvent solution of an adhesion promoter prepared by mixing an organosiloxane having the formula ##EQU3##
where A is vinyl or methyl and k is 0 to 20 and an acetoxysilicon compound, and optionally, an alkyl polysilicate and an organotitanate. While these compositions provide excellent adhesion between silicone elastomers and various substrates, the organosiloxane is expensive to prepare.
Bruner in U.S. Pat. No. 3,035,016 teaches a compound of the formula ##EQU4## where R and R' are monovalent hydrocarbon radicals, halogenated monovalent hydrocarbon radicals or cyanoalkyl radicals, Ac is a saturated aliphatic monoacyl radical of a carboxylic acid and n is an integer of at least 5, but does not teach an adhesion promoter or a primer composition.